Different types of immunotherapy have been shown to induce both immune and clinical responses in different malignancies, but at this point, only a small proportion of patients may benefit. The goal of this project is to develop more effective immunotherapy for prostate cancer. We have demonstrated that enhancing T lymphocyte responses with an antibody to immune inhibitory molecule CTLA4 can induce clinical responses in a small proportion of patients with castrate-resistant prostate cancer. We have examined whether this treatment could be combined with the cytokine granulocyte-macrophage stimulating factor (GM-CSF), which is also being studied as an immunotherapy for prostate cancers. In a phase I clinical trial, we have shown that the combination of CTLA4 blockade and GM-CSF treatment is safe and may lead to clinical responses at a higher rate. In order for CTLA4 blockade to be further developed clinically, we must determine whether combination immunotherapy indeed results in improved clinical outcome. In specific aim 1, we will perform a randomized phase II clinical trial where patients with hormone-refractory prostate cancer will be treated with either anti-CTLA4 antibody alone or anti-CTLA4 antibody in combination with GM-CSF. In the second aim, we will determine whether CTLA4 blockade alone or the combination leads to significant activation of effector T cells and expansion of regulatory T cells. In the final aim, we will define antigen-specific immune responses induced by the treatment. In doing so, we may validate novel tumor antigens that may represent novel vaccine candidates. Alternatively, we may define an immune response signature that may predict which patients might respond to this immunotherapeutic approach.